1. Field of the Invention
This invention relates generally to a method of determining the internal temperature of a battery and, more particularly, to a method of determining the internal temperature of a vehicle battery using a non-linear dynamic model.
2. Discussion of the Related Art
A typical internal combustion engine of a vehicle includes a battery, usually a 12-volt DC battery, that provides power to operate the various vehicle electrical systems. When the battery is providing power to one or more of the vehicle electrical systems, the power drain on the battery reduces the battery charge, and thus its voltage output. When the vehicle is running, a vehicle alternator recharges the battery so that the battery charge is high enough for continued power output to the vehicle electrical systems. The greater the power drain on the battery, the more charging voltage needs to be applied to the battery from the alternator. To control the charging output of the alternator, a field control input signal from a controller sets the duty cycle of the alternator depending on the drain on the vehicle battery. This field control of the alternator is referred to as the electronic voltage regulator (EVR) set point of a vehicle battery charging system.
Modern vehicles include an engine control module that provides vehicle system output control signals based on various inputs from the operation of the vehicle. Such an engine control module 10 is depicted in FIG. 1 and includes a central processing unit (CPU) 12. The CPU 12 receives various input signals, including engine speed, coolant temperature, manifold pressure, throttle position, etc., and outputs various output signals to control the operation of the vehicle, including fuel injector, ignition coil, idle speed motor, fan relay, etc., as is well understood in the art. One of those outputs is an alternator control output that provides the EVR set point.
Battery temperature is an important parameter for providing battery charging control. The temperature of the vehicle battery is required to determine an accurate EVR set point for proper battery charging. A proper set point for the desired battery target voltage will prevent the battery from overheating at high battery temperatures during charging, and prevent the battery from being undercharged at low battery temperatures during charging. For current vehicle technology, a battery temperature sensor, such as a negative temperature coefficient sensor, is mounted outside of the vehicle battery, usually under the battery tray that the battery is mounted on. Because the sensor is mounted outside of the battery, the temperature measurement is severely affected by the surrounding environment, and therefore does not reflect the dynamic variations of the battery's internal temperature very well. The battery temperature sensor thus, generally gives a poor temperature measurement performance. The inability to accurately measure a vehicle battery's temperature adversely affects the recharging of the battery. Errors between the real battery temperature measured with a temperature sensor inside the battery, and the temperature measured with a temperature sensor outside of the battery have been shown to be as high as 40.degree. F.
It is impractical to incorporate a temperature sensor within the vehicle battery itself because of the costs involved, the design requirements of the battery, and the fact that the battery is often replaced by the user. Additionally, not only does the temperature sensor mounted outside of the battery not provide an accurate temperature measurement of the internal temperature of the battery, it also adds cost for the sensor A/D device and installation labor to the vehicle. Therefore, it would be beneficial to eliminate the temperature sensor outside of the battery, and provide a more reliable and cost effective technique for determining battery temperature.
It is therefore an object of the present invention to provide such a technique of predicting the internal temperature of a battery.